1. Field of the Invention
This invention relates to a focal-plane shutter for digital still cameras which is designed to actuate a first blade and a second blade in turn in the same direction when photographing is performed and to expose an imaging surface through a slit provided by both blades.
2. Description of Related Art
In focal-plane shutters used in digital still cameras, some types of shutters are known in accordance with the specifications of cameras. One of them, like a focal-plane shutter used in a film camera, is equipped with a first blade and a second blade to actuate these blades in turn at predetermined timing so that an imaging surface is exposed continuously from its one side to the other through a slit provided by the blades.
Digital still cameras using shutters constructed as mentioned above are divided into two classes, those in which an optical finder is provided as in the film camera so that the imaging surface is covered by at least one of the first blade and the second blade, except for the case where photography is performed, and those, having finders referred to as electronic view finders, in which the entire surface in an exposure aperture is exposed to light from an object so that an image of the object can be observed through a monitor, except for the case where photographing is performed. Thus, the latter camera is such that, in photography, the first blade is moved to the position where it covers the imaging surface, and then an exposure operation is performed.
In the film camera, the area of the imaging surface is usually larger than that of the digital still camera. The film camera always has the optical finder. Due to the arrangement of an optical system and the placement of a film, each of the first blade and the second blade is composed of a plurality of blade components to achieve compactness of a shutter unit. However, in the digital still camera, particularly provided with the electronic view finder, each of the first blade and the second blade need not necessarily be constructed with a plurality of blade components. An example of a shutter constructed from such a viewpoint is shown in FIG. 1 (refer to Japanese Patent Preliminary Publication No. Hei 11-326992).
Focal-plane shutters are of two types. They are called a direct type and a locking type in accordance with the difference between systems for holding a first-blade driving member and a second-blade driving member at an exposure operation starting position. The direct type shutter is designed so that each of the driving members is attracted and held directly at the exposure operation starting position by an electromagnet energized after the release of a camera, and the exposure operation is performed immediately when the electric conduction of the electromagnet of each driving member is interrupted in turn. Therefore, in this type, after a cocking operation, a cocking member cannot be restored immediately to an initial position before cock, and after the electromagnet is energized prior to subsequent photography, the cocking member is restored. The shutter shown in FIG. 1 is constructed as such a direct type shutter.
On the other hand, the locking type shutter is such that each driving member is mechanically held at the exposure operation starting position by a corresponding locking member, and when the conduction of the electromagnet of each driving member is interrupted, a release member separated from the electromagnet releases a locking state of the driving member to perform the exposure operation. Hence, in the shutter of this type, the cocking member may be restored to the initial position immediately after the cocking operation, or it may be restored prior to photography, as in the direct type shutter, in association with the release of the camera.
Since the focal-plane shutter for digital still cameras mentioned above, in contrast with that for film cameras, is not subjected to severe restriction of space, each of the first blade and the second blade, as described above, can be constructed with a single blade component. However, if the entire shutter unit is enlarged, the number of degrees of camera design freedom will be limited in inverse proportion to shutter size, and the possibility of using this shutter unit will diminish. Thus, apart from the number of blade components of each of the first blade and the second blade, at least, compactness of the entire shutter unit is the maximum requirement, together with cost reduction thereof.
When the conventional example shown in FIG. 1 is considered from such a standpoint, the shutter is constructed so that the vertical dimension of a base plate 101 must be substantially increased. Specifically, in FIG. 1, a second blade 105 is located at a higher position than a first blade 102, and when the exposure operation is performed, the second blade 105 passes the place of the first blade 102 and then blocks an optical path AP. Thus, the conventional example, in contrast with an ordinary shutter, unnecessarily provides a space for the second blade 105 and must increase the vertical dimension of the base plate 101 accordingly.
In order to solve this problem, it is conceivable that, in FIG. 1, the second blade 105 is located at the same position as the first blade 102 so that after the first blade 102 completely blocks the optical path AP and then starts to open it, the actuation of the second blade 105 is begun. In this case, however, a large difference arises between time required for the first blade 102 from the start of blade actuation to the start of opening of the optical path AP and time required for the second blade 105 from the start of blade actuation to the start of closing of the optical path AP. Consequently, it is extremely difficult to provide a proper slit for making proper exposure.
Specifically, as is well known, the first blade 102 and the second blade 105 are actuated by their respective driving springs. It is, of course, desirable that these driving springs are identical parts. In the above construction, however, the first blade 102 and the second blade 105 have a large difference in time between the start of blade actuation and the start of crossing of the optical path AP by the edge of the slit. Thus, there is a considerable difference between both in biasing force of the driving spring exerted when the edge of the slit crosses the path. Consequently, a large difference arises between the blades 102 and 105 in speed (operation characteristic) where the optical path AP is crossed, and it is impossible to maintain a proper slit from start to finish in the exposure operation. Although the use of driving springs with different specifications is considered, it is very difficult that the shutter is designed to across the optical path AP at the same speed (operation characteristic).
In the conventional example mentioned above, even though each of the first blade 102 and the second blade 105 is constructed with a plurality of blade components, it is impossible to obtain further compactness of the shutter unit. Specifically, for example, if the first blade 102 and the second blade 105 are constructed with respective two blade components and are arranged as in FIG. 1 in a state where the respective two blade components are completely superimposed, the upper dimension of the optical path AP will be approximately halved. In this case, however, the two blade components of the first blade, after blocking the optical path AP in a state where the amount of mutual superposition is minimized, are moved downward by the same amount as in this state, and thus the dimension below the optical path AP is required twice. As a result, the upper and lower dimensions of the entire shutter unit cannot be reduced.